Blow molding makes hollow parts by inflating a hot tube of plastic against a mold cavity. It is how nearly every plastic bottle is made and how a great many ducts, tanks and hollow structural parts are made too. Comparing it sensibly with rotational and injection molding is a question about wall control, part size and volume rather than about the parts themselves.
How the process works
A parison, a hot tube of plastic, is either extruded continuously or molded first and reheated. The mold closes around it and air inflates it against the cavity walls. The part cools, the mold opens and the flash at the pinch off is trimmed. Extrusion blow molding suits handled containers and technical parts; injection stretch blow molding produces the clear bottles.
Against rotational molding
Rotomoulding tumbles powder inside a heated mold so the material coats the walls evenly, giving very uniform thickness, stress free parts, very cheap tooling and very long cycle times. Blow molding is far faster and gives less uniform wall thickness. Large tanks and low volume hollow parts go to rotomoulding; bottles and high volume hollow parts go to blow molding.
Blow molding vs injection molding
Injection molding forms both faces of a part precisely and cannot make a closed hollow shape in one piece. Blow molding makes the hollow shape but controls only the outside surface, with the inside wall thickness varying with how the parison stretched. Where a hollow part needs precise internal features, it is usually injection molded in halves and joined.
Designing for it
Keep the shape as close to uniform in cross section as the function allows, because sharp changes cause thin walls. Provide generous radii for the same reason. Design the pinch off line so the trimmed seam falls somewhere acceptable. And accept that the internal wall thickness is a consequence of the process rather than a dimension you specify directly.
Questions people ask about blow molding vs rotational molding
What is blow moulding?
Inflating a hot tube of plastic against a mold cavity to make a hollow part, then trimming the flash at the pinch off.
Blow moulding or rotational moulding?
Rotomoulding for large, low volume, stress free hollow parts with very cheap tooling. Blow molding for high volume and much faster cycles.
Why can injection moulding not make a bottle?
Because it forms both faces against tooling and cannot produce a closed hollow shape in one piece. Hollow injection molded parts are made in halves and joined.
Can I specify the internal wall thickness?
Not directly. It results from how the parison stretched, so it is controlled by shape, parison programming and process rather than by a dimension on the drawing.